Hello Ruchi,
Every EDI Format would imply a corresponding XSD generation and structure creation in PI.
Canonical Models would have made sense if your EDI 850 message is going to be consumed by different down stream applications but as you have mentioned in your case it is just to be integrated with ECC. In that case in my opinion your Orders05 Idoc is a canonical model in itself.
What I would do is build a template mapping with EDI 850 4010 to ORDERS05 and then leverage this mapping for other EDI 850 standards and so on.
If you are still keen on using a Canonical Model, you can explore the Global Data Types SAP provides and look up the Order creation data type but this would be a overkill in my opinion.
Regards
Bhavesh